One of the former Khmer Rouge's highest-ranking leaders has been freed from detention in Cambodia after a war crimes tribunal declared her unfit for trial.

Eighty-year-old Ieng Thirith was freed on a provisional release Sunday.

Thirith, one of only a handful of people ever brought before a court over atrocities during the Khmer Rouge era, is reported to have Alzheimer's disease, a form of dementia.

Prosecutors have appealed the decision to release the former social affairs minister.

Until the appeal is heard, Thirith must remain in Cambodia, inform authorities of her address and report to the court whenever it summons her.

She faced an array of charges in the deaths of some 1.7 million Cambodians during the Khmer Rouge rule of the 1970s.